Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer(s) and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell changes due to the orientation of the magnetic fields of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.
Spin transfer torque or spin transfer switching, uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction (“MTJ”). In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer thus produces a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer in the MTJ device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel states relative to the reference layer.
When a current is passed through a magnetic layer (e.g., a polarizer), the spin orientation of the electrons that flow out of the magnetic layer is generally aligned in the direction of the magnetization of the magnetic layer and will exert a spin-transfer torque in that direction (forming a transverse spin current) upon impinging on another magnetic layer. However, due to the conservation of angular moment for the system, the electrons on the opposite side of magnetic layer, those that do not go through the magnetic layer, generally have a spin orientation that is aligned in the direction that is anti-parallel to the magnetization direction of the magnetic layer. The net effect of this process is that the current applied to the magnetic layer undergoes spin filtering, which creates a spin current on one side of the magnetic layer, with spins that are aligned with magnetization direction of the magnetic layer, and a reflected spin current on the other side of the magnetic layer, with spins that are anti-parallel to the magnetization direction of the magnetic layer. This effect occurs upon application of a current to any magnetic layer, including an in-plane polarization layer or an out-of-plane reference magnetic layer. Thus, in a typical MTJ, when switching the magnetization direction of the free layer in one direction (e.g., from the parallel to anti-parallel state) is achieved using spin transfer torque from the transverse spin current, switching the free layer in the other direction (e.g., from the anti-parallel to parallel states) would be achieved using spin transfer torque from the reflected spin current. This is typically accomplished by running electrical current through the MTJ in one direction when switching from the anti-parallel to parallel state and running the electrical current through the MTJ in the other direction when switching from the parallel to anti-parallel state.
FIG. 1 illustrates a MTJ stack 100 for an MRAM device including a magnetic tunnel junction MTJ 130 and a top polarizer layer 150. As shown, stack 100 includes one or more seed layers 110 provided at the bottom of stack 100 to initiate a desired crystalline growth in the above-deposited layers. Furthermore, MTJ 130 is deposited on top of synthetic antiferromagnetic (SAF) layer 120. MTJ 130 includes reference layer 132, which is a magnetic layer, a non-magnetic tunneling barrier layer (i.e., the insulator) 134, and the free layer 136, which is also a magnetic layer. It should be understood that reference layer 132 is actually part of SAF layer 120, but forms one of the ferromagnetic plates of MTJ 130 when the non-magnetic tunneling barrier layer 134 and free layer 136 are formed on reference layer 132. As shown in FIG. 1, reference magnetic layer 132 has a magnetization direction perpendicular to its plane. As also seen in FIG. 1, free layer 136 also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees.
The first magnetic layer 114 in the SAF layer 120 is disposed over seed layer 110. SAF layer 120 also has an antiferromagnetic coupling layer 116 disposed over the first magnetic layer 114. Furthermore, a nonmagnetic spacer 140 is disposed on top of MTJ 130 and a polarizer 150 is disposed on top of the nonmagnetic spacer 140. Polarizer 150 is a magnetic layer that has a magnetic direction in its plane, but is perpendicular to the magnetic direction of the reference layer 132 and free layer 136. Polarizer 150 is provided to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure 100. Polarizer 150 polarizes the current in a direction perpendicular (orthogonal) to those of the magnetizations of the free magnetic layer 136 and reference magnetic layer 132. Further, one or more capping layers 160 can be provided on top of polarizer 150 to protect the layers below on MTJ stack 100. Finally, a hard mask 170 is deposited over capping layers 160 and is provided to pattern the underlying layers of the MTJ structure 100, using a combination of reactive ion etch (RIE) and ion beam etching (IBE) processes.
Various mechanisms have been proposed to assist the free-layer magnetization switching in MTJ devices. One issue has been that to realize the orthogonal spin transfer effect for in-plane MTJ structures, large spin currents may be required for switching. The need for large switching currents may limit such device's commercial applicability. One way proposed to reduce switching current is to lower the magnetization of the free layer. However, if the effective magnetization of the free layer is lowered significantly, the orthogonal effect has to be limited so that the free-layer does not go into precessional mode that would make the end state of the free-layer magnetization un-deterministic. This defines the operation window for the in-plane OST structures. In an in-plane device, unlike that shown in FIG. 1, the magnetization direction of the reference layer and free layer are in the plane of the layer. Another aspect of in-plane devices is that the thermal stability requirements may limit the size of the MTJ devices to approximately sixty nanometers or higher.
In contrast to MTJ structures with an in-plane free layer and a perpendicular polarizer, perpendicular MTJ structures, such as those shown in FIG. 1, are less prone to getting into a pure precessional regime. This is due to the fact that in perpendicular MTJ structures, the direction of the demagnetization field and perpendicular anisotropy contributions are the same. In this case the precession is not an issue and makes the end-state more deterministic. However, precession may be an issue with regards to read disturb, particularly when stronger read currents are used. The orthogonal polarizer acts on the free layer magnetization at the initial state, but when the precession takes hold, the fixed orthogonal polarizer 150 helps only half the cycle of the free-layer magnetization rotation while it harms the other half of the cycle. This is demonstrated with reference to FIGS. 2-3. FIG. 2a-2b shows switching of a free layer 136 of an MTJ. As is seen, free layer 136 has a magnetization direction 200 perpendicular to that of the polarizer 150. The magnetization direction 200 of the free layer 136 can rotate by 180 degrees. FIGS. 2a-2b show precession about the axis of the magnetization vector of free layer 136. During precession, magnetic vector 200 begins to rotate about its axis in a cone-like manner such that its magnetization vector 200′ deflects from the perpendicular axis 202 of free layer 136. Whereas prior to initiating precession, no component of magnetic vector 200 is in the plane of free layer 136, once precession starts, a component of magnetic vector 200′ can be found both in-plane and orthogonal to free layer 136. As magnetic vector 200′ continues to precess (i.e., switch), the rotation of vector 200′ extends further from the center of free layer 136, as is seen in FIG. 2b. 
In most prior MTJ devices using a polarizer such as polarizer 150, the magnetization direction of polarizer 150 is fixed, which is shown in FIGS. 1 and 3. See also U.S. Pat. No. 6,532,164, which states that the direction of the magnetization of the polarizing layer cannot vary in the presence of current. Prior to current passing through the MTJ, the free layer 136 has a magnetization direction 200 perpendicular to that of the polarizer 150. While the magnetization direction 200 of the free layer 136 can rotate by 180 degrees, such rotation is normally precluded by the free layer's inherent damping ability 205, which is represented by a vector 205 pointing to axis 202 (shown as a dashed line in FIG. 2a as well as FIG. 3). Axis 202 is perpendicular to the plane of free layer 136. This damping 205 has value, defined by the damping constant, which maintains the magnetization direction of the free layer 136.
The precession of the magnetization vector during switching of the free layer can be assisted by spin transfer torque exerted by the electrons of a spin-polarized current, which is generated in part by the orthogonal polarizer 150. Applying a voltage across MTJ device 100 produces a charge current that flows through the structure. Application of this current to the MTJ device 100, in turn, produces a spin-polarized current, which exerts a spin transfer torque on the magnetic vector 200. This spin transfer torque has an in-plane component of the spin transfer torque 210, which pushes magnetization vector 200′ in the direction of the magnetic vector of polarizer 150 throughout precession of magnetic vector 200′. In addition to the in-plane spin transfer torque 210 from the polarizer, the perpendicular spin transfer torque (not shown), generated by reference layer 132, pulls the magnetic vector 200′ towards the direction antiparallel to its initial direction 200, thereby causing switching of the free layer 136. In devices like those shown in FIG. 1, when the spin transfer torque 210 begins to help overcome the damping 205 inherent to the free layer 136, the magnetic direction 200′ begins to precess about its axis, as shown in FIG. 2a. As seen in FIG. 3, in-plane spin transfer torque 210 helps the magnetization direction of the free layer 136 to precess in a cone-like manner around an axis 202 perpendicular to the plane of the layers. When a spin polarized current traverses the stack 100, the magnetization of the free layer 136 precesses in a continuous manner (i.e., it turns on itself in a continuous manner as shown in FIG. 3) with maintained oscillations until the magnetic direction of free layer 136 is opposite the magnetic direction prior to the spin torque causing precession, i.e., the magnetic direction of free layer 136 switches by 180 degrees.
FIG. 3 illustrates precession of a free layer 136 of an MTJ assisted by orthogonal spin polarized current. The spin polarized electrons from polarizer 150 provide a spin transfer torque which has a component 210 in the plane of the precession (i.e., in-plane spin transfer torque) that helps overcome the damping 205 in the first half of the precession 215 because the in-plane spin transfer torque 210 provided by the spin polarized current is opposite that of the inherent damping 205 of the free layer 136. This is shown on the right-hand side of the middle portion of FIG. 3. However, the in-plane spin transfer torque actually harms the switching process during the second half of the precession 220. The reason for this is that the spin of the electrons in the spin polarized current only apply an in-plane spin transfer torque 210 in the direction of their polarization, which is aligned with the magnetic direction of the in-plane polarization layer 150. Thus, when the magnetic vector is in the half of the precession cycle 220 that is opposite the spin of the polarized electrons, the in-plane spin transfer torque 210 actually works with the inherent damping 205 of free layer 136 to make rotation more difficult. This is shown in the left-hand side of the middle portion of FIG. 3 and can be seen in the projection of the spin transfer torque 210 onto the precessional plane of the free layer 136, which is depicted on the bottom of FIG. 3. Indeed, it is the perpendicular spin transfer torque created by the reference layer 132 (not shown in FIG. 3) that overcomes the damping 205 of free layer 136 as well as the in-plane spin transfer torque 210 during the half of a precession cycle where the spin of the electrons harms precession, and thus it is the reference layer 132 that allows for completion of precession. The precessional dynamics and the directionality of the spin transfer torque depicted in FIG. 3 are merely approximations at small magnetization polar angles and do not necessarily reflect the phenomena occurring at larger magnetization polar angles. However, the precessional dynamics that occur when the magnetization vector of the free layer 132 is at small magnetization polar angles are, to a large extent, determinative of the efficiency of the switching process.
One solution that has been proposed to overcome this limitation is the use of a precessional spin current (“PSC”) magnetic layer having a magnetization vector that can freely rotate in any magnetic direction, shown in FIG. 4a-b. The free layer 336 is similar to the free layer 136 previously discussed, in that it has an inherent damping characteristic 205 that can be overcome with the assistance of spin transfer torque. However, the polarizing layer 150 is replaced with a precessional magnetic layer 350. As seen in FIG. 4a, which shows the projection onto the precessional plane (the plane defined by the precession of vector 200′ as it precesses around axis 202) of the direction of the spin transfer torque 211 created by spin current passing through free layer 336, the direction of spin transfer torque 211 changes with the rotation of PSC magnetic layer 350. As seen on the right side of FIG. 4a, spin transfer torque 211 causes the magnetization direction 200′ of the free layer 336 to precess in a cone-like manner around an axis 202 perpendicular to the plane of the layers. The PSC layer 350 and the free-layer 336 are magnetically and/or electronically coupled such that the magnetization direction of the magnetization vector 270 of the PSC magnetic layer 350 follows the precessional rotation of the magnetic vector of the free layer 336.
As seen in on the right-hand side of FIG. 4a, the spin polarized electrons provide torque 211 that helps to overcome the damping 205 in the first half of the precession 215 because the torque 211 provided by the spin polarized current is opposite that of the inherent damping 205 of the free layer 336. In addition, torque 211 helps to overcome the damping 205 in the second half of the precession 220 by the same mechanism. Thus, unlike prior devices having a fixed polarization magnetic layer 150, the spin of the electrons in the spin polarized current applies a torque 211 in both halves of the precession cycle, including the half of the precession cycle 220 where devices with fixed polarization magnetic layers 150 actually harmed precession. As is seen, the torque 211 continues to help overcome the inherent damping 205 of free layer 136 throughout the entire precession cycle. An MRAM device utilizing an MTJ structure with a PSC is depicted in FIG. 5.
However, because of the chirality of perpendicular MTJ structures that utilize a PSC, such as the structure shown in FIG. 5, the PSC only enhances switching of the free layer in one direction (i.e., from the parallel state to the anti-parallel state, FIG. 4a), but not the other (i.e., from the antiparallel state to the parallel state, FIG. 4b). As discussed above, when switching the free layer 336 from the first direction to the second direction, the spin current is generated by the electrons passing through the PSC layer and the in-plane spin transfer torque 211 is in line with the magnetic vector of the PSC layer (FIG. 4a). However, during switching free layer 336 from the second direction to the first direction, it is the reflected spin current from PSC layer that imparts the in-plane spin transfer torque 211′ on the free layer 336. As shown in FIG. 4b, the in-plane spin transfer torque 211′ caused by the reflected spin current is in the direction anti-parallel to the magnetic vector 270 of the PSC layer 350. When the magnetic vector 270 is aligned with the magnetic vector 200′, the in-plane spin transfer torque 211′ might actually enhance the damping characteristic 205 of the free layer 336. Therefore, when the precession of magnetic vector 270 of the PSC layer 350 is synchronized with the precession of the magnetic vector 200 of the free layer 336, the in-plane spin transfer torque 211′ might enhance the damping characteristic 205 throughout the entire precession 220′. Thus, the PSC layer can be highly effective at increasing the switching efficiency of the free layer in one direction, but may actually hamper switching in the other direction.
Thus, in prior devices that utilize in-plane polarization layers having a fixed magnetization direction and having a free magnetic layer 150 that is perpendicular to the plane of the device, once the precession holds, the in-plane spin transfer torque has no net positive effect on the switching mechanism for a full three hundred sixty degree precession. Moreover, in prior devices that utilize a PSC magnetic layer, the in-plane spin transfer torque enhances the switching of the free layer throughout the precession from the first direction to the second direction, but might not enhance the switching of the free layer from the second direction to the first direction.
Therefore, there is a need for a spin torque transfer device that reduces the amount of current needed for switching from both magnetization directions while also switching at high speeds and requiring reduced chip area.